HostEnvironmentDirectedGraph::Iterator

[Robadob]

Branch named host_graph_traverse because I incorrectly thought this feature request was tree traversal for some reason, and created the branch before re-reading the issue.

Required some modifications to HostEnvironmentDirectedGraph::EdgeMap::Edge and CUDAEnvironmentDirectedGraphBuffers, as these were previously heavily written for Edges to be referred to by src/dest, rather than index, despite this requiring the index to be looked up by most methods.

Reason this probably wasn't implemented initially is because the index of an edge is not stable, as the edge list is sorted each time the map is built. Likewise, it iterates the buffer of allocated edges, rather than the named EdgeMap so the iterator does not match expectations of a map iterator (e.g. doesn't return key-value pairs) and leads to some ambiguity (e.g. does size() return the length of the allocated buffer, or the number of valid edges?).

• Edge iterator
• Vertex iterator (this would make sense too presumably)
• Implement C tests
• Implement Python tests
• docs Document HostEnvironmentDirectedGraph iteration FLAMEGPU2-docs#183

Closes #1237

[ptheywood]

TestEnvironmentDirectedGraph tests for c++ and python all pass under linux.

Changes look sensible to me, no API breaks I spotted (and we're not concerned with ABI).

Will let @mondus have the final say.